What will happen to letters if dns records are temporarily changed?

Question

Mail for domain

Good afternoon, tell me what will happen, we use mail for the Yandex domain, everything works. By mistake, all records were changed, mail stopped working, then the records were restored.
Question: what will happen to those letters that were sent to our boxes? How can we find them?

Answer 1

They will not be delivered, they will be "returned" to the sender. If the mail server cannot send mail due to unfound dns records, the letter is "returned" back.